Liquid crystal displays (LCDs) have become widespread. Such LCDs have low power consumption and thin-profile, and are used in various devices ranging from small devices such as a monitor of a notebook computer, a digital camera and a mobile phone to large devices such as a home TV. Backlight LCDs whose LCD panel is illuminated with a backlight are commonly used.
Generally, the backlight LCD is composed of an LCD panel and a light source unit (backlight). The LCD panel changes light transmittance in response to an electric signal. The light source unit irradiates illumination light from the back of the LCD panel. The LCD panel includes a pair of polarizing filters and a liquid crystal cell. The polarizing filters are arranged in cross nicols. The liquid crystal cell is sandwiched between the pair of polarizing filters, and changes the polarizing state of the light passing therethrough. The light source unit includes a light source such as a fluorescent tube, a diffusion sheet and a brightness enhancement sheet. The diffusion sheet scatters and diffuses light from the light source to uniformly illuminate the entire surface of the LCD panel. The brightness enhancement sheet is a so-called BEF (Brightness Enhancement Film) that enhances front brightness.
A prism sheet is used as the brightness enhancement sheet as described in Japanese Patent Laid-Open Publication No. 10-082903, for example. One of the surfaces of the prism sheet is formed with a plurality of prisms, each having a triangular cross-section, with a predetermined pitch. The light incident from the surface opposite to the prism-formed surface is refracted and directed in the direction normal thereto through the prisms. As a result, the light distribution shows a large peak in the direction normal to the prism sheet. The illumination light enters the LCD panel through the prism sheet, and thus the front brightness of the LCD is enhanced. On the prism sheet, the prisms extend in one direction and arranged parallel to each other. The illumination light is output along a direction in which the prisms are arranged. For this reason, two prism sheets overlaid at an angle of 90 degrees to each other may be used.
Since prism apexes formed on the surface of the prism sheet are pointed, the prism sheet may frequently damage the surface of adjacent components, for example, the diffusion sheet and the LCD panel. In addition, projections and depressions formed by the prisms on the surface of the prism sheet increase the thickness thereof. For these reasons, the prism sheet requires a large spacing in the LCD and increases the number of components, resulting in cost increase. Thus, the prism sheet has been one of the factors that hinder the reduction of the profile of the LCDs.
To produce the prism sheet, UV curable polymer is applied to a roll or a mold on which a prism pattern is formed, and the roll or the mold is overlaid on the substrate, and cured by UV rays, for example. This production process has a limited production speed, resulting in low productivity. In addition, the production of the prism sheet requires extremely fine processing on the surface thereof, making the prism sheet expensive due to low yields.